“Spot welding” is the process of superposing two welded members, bringing columnar electrodes into contact with a weld location from the top and bottom, and applying pressure while running current so as generate Joule's heat by the resistance of the conduction path and using this to form a weld (below, “nugget”).
In recent years, to realize performance such as compliance with safety standards for vehicle bodies and reduction of the environmental load, high strength steel sheet having a 1500 MPa or more tensile strength is used.
However, for example, NPLT 1 reports that the tensile shear strength (TSS) of a spot welded joint tends to increase along with the strength of the steel sheet, but the cross tensile strength (CTS) of the spot welded joint falls when the strength of the steel sheet becomes 780 MPa or more. In high strength steel sheet with a strength of the steel sheet of 780 MPa or more, improvement of the cross tensile strength (CTS) of the spot welded joint is a critical issue. The spot welding electrode according to the present invention does not have to be limited to a steel sheet as the welded member, but in this Description, the welded member will be explained as a steel sheet.
It is known that the cross tensile strength (CTS) of a spot welded joint is improved if the nugget increases in outer circumferential length. R&D has been conducted to increase nugget size. However, the problem is faced that if trying to enlarge a nugget, the current required for the spot welding also becomes larger and as a result the spot welding apparatus also has to be made larger.
Therefore, R&D is underway for realizing a suitable cross tensile strength (CTS) of a spot welded joint by using a suitable size spot welding apparatus.
PLT 1 discloses a welding electrode designed to improve the cross tensile strength (CTS) of a spot welded joint by making the nugget a donut shape and thereby increasing the circumferential length without changing the area. Specifically, it discloses a spot welding electrode making the electrode a donut shape so as to enable the formation of a donut-shaped (ring-shaped) nugget and providing a cylindrical shaped hollow part so that the center does not contact the steel sheet.
FIG. 18 is a conceptual view of a spot welding apparatus having conventional spot welding electrodes, while FIG. 19 is a conceptual view of a conventional spot welding electrode.
In FIG. 18, 140 indicates a spot welding apparatus having conventional spot welding electrodes, 50 indicates steel sheets, and 100 indicates conventional spot welding electrodes.
In FIG. 19, a contact-and-conduction surface 105 of a contact-and-conduction part 104 contacting and conductive with the steel sheet is, for example, formed as a donut shape (circular ring shape) projection. By the projection contacting the steel sheet, a so-called donut-shaped nugget is formed. In the Description, an electrode having a donut shaped contact-and-conduction part will be called a “donut-shaped electrode”.
PLT 2 discloses a method of judging whether a nugget is suitably formed so as to manage the quality of spot welding. PLT 2 describes to perform spot welding using for at least one welding electrode among the pair of welding electrodes performing spot welding a welding electrode with an electrode surface of a substantially projecting curved surface and with a center of the electrode surface formed with a recess set to a predetermined depth whereby the weld formed in the metal surface reaches the deepest part when spot welding forms a good nugget and to judge that the spot welding zone is suitably formed with a nugget based on the fact of the resistance value at the time of welding becoming a balanced state.